Attempts have been made to apply graphene to a variety of products because of its excellent electric characteristics such as high conductivity and high mobility and its excellent physical characteristics such as sufficient flexibility and high mechanical strength (see Patent Documents 1 to 3).
Here, in a commercially available secondary battery, which is a rechargeable power storage device, a carbon material such as graphite is used for a negative electrode. Graphite has a crystal structure where sheets of carbons which have sp2 hybrid orbitals and are regularly arranged two-dimensionally are stacked. The secondary battery is charged and discharged utilizing occlusion of lithium ions from a positive electrode into a gap between sheets of carbons in the crystal structure.
Other than graphite, carbon materials which are suitable as negative electrode materials of lithium-ion secondary batteries have been researched (Patent Document 4). Such carbon materials contain carbon particles containing a metal which forms an alloy with lithium and have gaps between the carbon particles, in order that high lithium occlusion/release capacity and prevention of possibilities of damages due to continuous charge/discharge can be achieved.
A material which generates electricity by being directly involved in cell reaction is called an active material of a battery. Although it is preferable to reduce the particle size of an active material of an electrode in terms of higher power of a secondary battery, when the particle size is reduced, a conductive additive and a binder are needed to efficiently construct a conductive network. However, a conductive additive and a binder decrease capacity per unit volume of an electrode, which is a problem. Therefore, a battery which does not include a conductive additive and a binder and in which an active material is supported by whiskers between an active material layer and a surface of a current collector has been studied (Patent Document 5). In Patent Document 5, as examples of active materials, lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, lithium iron phosphate, lithium titanate, graphite, and hard carbon are given, and as examples of whisker materials, carbon, potassium titanate, titanium carbide, silicon carbide, titanium dioxide, zinc oxide, magnesium oxide, tin dioxide, and indium oxide are given.
It is known that occlusion of lithium released from a positive electrode expands and, further, crushes a negative electrode. To solve this problem, a structure where particles of a metal or a semimetal which form a lithium alloy serve as cores and the cores are coated with carbon has been suggested (Patent Document 6). Patent Document 6 discloses that silicon particles are preferred as particles serving as cores and a chemical vapor deposition method is employed as a carbon coating method.